Vry interesting thread, I enjoyed reading about the mechanic that thought conventional gear oil allowed junk to "stick" to the differential housing. I guess he wanted to completely disregard the fact that GM calls for a synthetic 75w-90 gear oil by name.

I am seeing at of people that swear that you should add LSD to the oil as it will help the G80's clutches. The G80 is a locker, not a limited slip.

The G80 parts diagram I've looked at make it look like it is basically a clutch style LSD with the ability to force the the clutches together when it senses slip between the two wheels. It actually doesn't lock in the sense a true locking differential does (like a Detroit Locker, which has no clutches).

On my Colorado we just swapped in a 2006 Trailblzer SS rear axle assembly. 9.5 Eaton locker 4.10 33 spline axle. We used Mobil 1 75w 90 oil and we added 4 ounces of the additive. The dealer actually gave me the TSB where it states that (this particular) rear axle can develop "howling", and early wear on the clutches if the additive is not used. He was nice enough to let me know about it, so I followed his advice per the TSB.

the additive of the friction modifier has been not recomended in the past because it will damage the G80.

post the TSB # or reproduce this here.

the only issues I have found with a new eaton G80 is it requires a breakin period with light loads for the first 1000mi. then after 2,ooo mi the rear cover is to be removed the gearing teeth inspected for proper meshing and the boxed cleaned.

I have read many documents with the G80 warning not to use any friction modifier additive.

If you drive this vehicle hard the first 1000mi the gear box will get very hot and then the teeth will be brittle and crack and break off. no loads as well..

The use of any additive in locking rear axles (G80) is not recommended. Rear axle additives are designed for use in limited slip differentials which are normally installed in cars. All light duty trucks equipped with RPO G80 make use of a locking differential and the use of additives will delay the engagement of the locking mechanism and may decrease axle life.

as you can see a clean box causes the diff to work as designed. issues arise when it is not clean. this causes delay in engagement which wears out the diff.

also no additive as this will damage the G80 diff.
SAE 75W-90 Synthetic Axle Lubricant, GM P/N 12378261, this is the correct gear oil to use . the other gear oil stated by GM was to correct chatter issues IF the box was not dirty and still chattered.

On my Colorado we just swapped in a 2006 Trailblzer SS rear axle assembly. 9.5 Eaton locker 4.10 33 spline axle. We used Mobil 1 75w 90 oil and we added 4 ounces of the additive. The dealer actually gave me the TSB where it states that (this particular) rear axle can develop "howling", and early wear on the clutches if the additive is not used. He was nice enough to let me know about it, so I followed his advice per the TSB.

Click to expand...

The Trailblazer SS trucks do not use the G80, they use a true LSD diff, additive is probably a smart move.

Replace the rear differential center section (gear case) which will inlcude clutch plates and springs with an updated spring rate. At the time of the release of this PI all parts in GMSPO stock have been purged and ordering the catalog listed part number will insure updated parts. Fill the differential by first installing 5.5 oz (163 mL) of Limited-Slip Axle Lubricant Additive (GM Part No. U.S. 1052358) then fill, 0.6 in. to 1.6 inches below th ebottom of the filler plug hole using SAE 75W-90 Synthetic Axle Lubricant (p/n 12378261) meeting GM Spec 9986115.

Note 1: reference the latest version of tsb 06-04-20-003 for proper fluids and capacity.

Note 2: Synthetic Axle Lubricant, p/n 12378261 or p/n 89021677 each of these part numbers is the same axle lubricant, w/ differences in packaging only.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved w/o completing every step, the remaining steps do not need to be performed.